1. Field of the Invention
This invention relates to electrochemical H.sub.2 S spontaneous conversion to S and H.sub.2 O with the concurrent production of electrical energy. The electrochemical H.sub.2 S conversion is suitable to a wide variety of H.sub.2 S containing gas cleanup processes, such as in gasification and liquefaction of naturally occurring carbonaceous materials.
2. Description of the Prior Art
Large quantities of H.sub.2 S are generated in gasification and liquefaction of naturally occurring carbonaceous materials, such as coal, and in heavy oil desulfurization. Typically, H.sub.2 S is removed from product streams by absorption of H.sub.2 S in a solvent and the dissolved H.sub.2 S stripped from the solvent. The H.sub.2 S is generally then subjected to an oxidation process to form elemental sulfur and water. The Claus process is a well known, commonly used industrial process for the chemical conversion of H.sub.2 S to elemental sulfur. However, due to stringent air pollution regulations currently in effect, SO.sub.2 containing tail-gases must be further treated to reduce or eliminate the sulfur content of the gas entering the atmosphere as taught by Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 22, pgs. 276-293, Sulfur Recovery. The Claus process results in heat available for other uses.
The thermal dissociation of H.sub.2 S in the presence of a suitable catalyst and separation of hydrogen by selective ceramic diffusion membranes is taught by Kameyama, T., M. Dokiya, F. Fujishige, H. Yokokwawa and F. Fukuda, Int. J. Hydrogen Energy, "Production of Hydrogen from Hydrogen Sulfide by Means of Selective Diffusion Membranes", 8, 5-13 (1983).
Indirect H.sub.2 S conversion to elemental S in electrochemical cells requiring electric energy input is described in Kalina, D. W., E. T. Maas, Jr., Int. J. Hydrogen Energy, "Indirect Hydrogen Sulfide Conversion - I. An Acidic Electrochemical Process", 10, 157-162 (1985); and Kalina, D. W., E. T. Maas, Jr., Int. J. Hydrogen Energy, "Indirect Hydrogen Sulfide Conversion - II. A Basic Electrochemical Process", 10, 163-167 (1985).
In addition to being an undesired atmospheric pollutant, the presence of H.sub.2 S has been found to result in degradation of electrochemical performance of fuel cells, including molten carbonate fuel cells as taught by Sammells, A. F., S. B. Nicholson and P. G. P. Ang, J. Electrochem. Soc., "Development of Sulfur-Tolerant Components for the Molten Carbonate Fuel Cell", 127, 350-357, (1980), and solid oxide fuel cells as taught by Fuel Cells Technology Status Report, DOW/METC-86/0241, Morgantown Energy Technology Center (1985).
The anodic oxidation of H.sub.2 and H.sub.2 S on yttria stabilized zirconia solid oxide electrolytes using Au, Pt or Ni electrodes wherein current darkened electrolyte produces trapped electrons or colored F-centers in the electrolyte which act as active electrocatalytic sites for the H.sub.2 S oxidation with the electrode playing an insignificant role in the catalytic process is taught by Ong, B. G., T. A. Lin and D. M. Mason, Abstract #531, "The Anodic Oxidation of H.sub.2 and H.sub.2 S on Yttria-Stabilized Zirconia (YSZ) with Porous Au, Ni, or Pt Metal Electrodes", Electrochem. Soc. Meeting, Philadelphia, Pa., May (1987).
Electrochemical membrane cells have been suggested for removal of H.sub.2 S from a hot gas stream. High temperature molten sulfide electrolytes with porous carbon electrodes provide that the process gas pass through the cathode chamber selectively removing H.sub.2 S and with passage of current, the anion migrates to the anode where elemental sulfur is removed are taught by Lim, H. S. and J. Winnick, J. Electrochem. Soc., "Electrochemical Removal and Concentration of Hydrogen Sulfide from Coal Gas", 131, 562-568 (1984).